Digital map data include, as a rule, road-edge data and road-node data, in order to reproduce a road network. A road edge represents, within the road network, a road segment between two successive intersections or junctions. A road node represents an intersection or a junction in the road network.
The graphical representation of a road network consists substantially in the concrete visual reproduction of the road contour of individual roads of the road network. A similar statement applies to the representation of a calculated route in the course of a route-calculation function in which the road contour of the calculated route is represented (wholly or piecewise). The road contour describes the curve contour of roads, road segments or routes within a cartographic coordinate system.
For the purpose of describing the road contour, so-called shape points have been saved in the digital map data. Each road segment (or each road edge) has in this case been associated with a succession of shape points that describe the curve contour of the road segment. Each shape point represents a position with respect to a cartographic coordinate system (e.g. geographical longitude and geographical latitude).
In current representation methods, use is made of the shape points assigned to the road segments or routes for the purpose of graphical representation of road segments or routes. The road contour is represented in accordance with the positions of the shape points, the road contour being linearly interpolated between respectively adjacent shape points.
An example of such a graphical representation of road segments by means of linear interpolation between adjacent shape points are shown in FIG. 1A and FIG. 1B. FIG. 1A shows a detail of a road network consisting of the road segments 60, 63, 64, which each terminate at the node 62. Since the road segments exhibit a strong curvature, the curve contours linearly interpolated between the shape points 60a, 60b, 63a, 63b, 64a and the nodal point 62 result in transitions that are angular and not smooth. As is immediately evident from the right-hand representation shown in FIG. 1A, striking transitions arise at points 60b, 62, 63a, 64a, which reproduce the real curve contour in greatly distorted manner.
One possible way to make the curve contour appear smoother consists in providing further shape points, in order to reduce further the spacings of adjacent shape points, as has been represented schematically in the left-hand drawing shown in FIG. 1B. In this connection, additional shape points (represented by the small circles) were inserted between the existing shape points 60a, 60b, 63a, 63b, 64a, 64b. The curve contour can in fact be represented more smoothly, and consequently more realistically, as is immediately evident from the right-hand representation in FIG. 1B.
The disadvantage of a solution of such a type, however, is that the information to be provided in the map data increases further, as a result of which still more memory space is needed. Furthermore, with increasing number of shape points the processing-time per frame to be represented increases, since in the course of the graphical representation each shape point has to be processed separately. This happens at the expense of the speed of graphical representation. However, a slow graphical representation is not desirable, since, for a liquid representation, map data have to be graphically updated, as a rule, several times per second. For performance reasons, therefore, a smaller number of shape points is chosen, and consequently an inferior quality of the graphical representation of map data is accepted.